A student cracks decane (C₁₀H₂₂) in the laboratory. The products include octene (C₈H₁₆) and one other substance. The student tests both products with bromine water. Explain what the student would observe in each test and why, and name the other substance produced during cracking.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Decane is a long-chain alkane that can be cracked to produce smaller, more useful molecules. One product of cracking decane is octene (C₈H₁₆).
Model answer (5 marks)
Octene decolourises bromine water, turning the orange solution colourless because it contains a carbon–carbon double bond that reacts with Br₂.
The other product does not decolourise bromine water; the orange colour remains because it is an alkane (saturated) with no double bond.
The other substance produced is ethane (C₂H₆).
The other product does not decolourise bromine water; the orange colour remains because it is an alkane (saturated) with no double bond.
The other substance produced is ethane (C₂H₆).
Examiner tips
- Use the command word ‘explain’ – describe the observation and give the reason. Mention the double bond for octene and the lack of a double bond for ethane. State the product’s identity explicitly.
- Use the exact terminology: ‘decolourises’, ‘unsaturated’, ‘saturated’, ‘alkane’.
- Include the chemical formula of ethane to show full credit.
Common mistakes
- Writing ‘octane’ instead of ‘octene’. Claiming the other product is a different alkene. Omitting the reason that the alkane does not react with bromine water.
Mark scheme (5 marks)
- Octene decolourises / turns bromine water from orange to colourless
- Octene decolourises bromine water because it is unsaturated / has a carbon–carbon double bond
- The other product does not decolourise bromine water / bromine water stays orange
- The other product does not react with bromine water because it is an alkane / is saturated / has no double bond
- The other substance produced is ethane (C₂H₆)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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